1. Field of the Invention
The present invention relates to a method and an apparatus for film thickness measurement in which the thickness of a thin film is measured based on the X-ray diffraction method.
2. Description of the Related Art
It is known to use X-rays in measuring nondestructively the thickness of a thin film. The typical X-ray-related method in the determination of a film thickness is to measure an X-ray reflectance. The X-ray reflectance method can determine not only the film thickness of a single layer but also each film thickness of two laminated layers composed of different materials as disclosed in, for example, Japanese patent publication No. 10-38821 A (1998) which will be called as the first publication hereinafter.
The film thickness measurement method based on the X-ray reflectance, however, can hardly determine a film thickness in the case that (1) the density of an object thin film to be measured is close to the density of a layer adjacent to the object thin film or (2) the boundary between the object thin film and the adjacent layer has a rough surface. Assuming that a tantalum layer is deposited on a tantalum nitride layer for example, since the densities of the two layers are very close to each other, the intensity of the reflected X-ray from the boundary of Ta/TaN is weak so that it is impossible to measure the film thickness of the tantalum layer in the X-ray reflectance method.
Then, it can be planned to measure a film thickness based on the diffracted X-ray intensity with a sample whose film thickness can be hardly measured in the X-ray reflectance method. Some methods about film thickness measurement based on the diffracted X-ray intensity are known: see for example Japanese patent publication No. 4-194611 A (1992) called as the second publication hereinafter, Japanese patent publication No. 10-103942 A (1998) called as the third publication hereinafter, Japanese patent publication No. 2000-88776 A (2000) called as the fourth publication hereinafter.
The second publication introduces that, in the description of the prior art, a film thickness can be measured based on a ratio of the first intensity of a diffracted X-ray from the object thin film to be measured to the second intensity of a diffracted X-ray from the foundation layer beneath the object thin film, the ratio being called as a diffraction intensity ratio hereinafter. This method includes the steps described below. Plural object thin films are prepared with known and different film thicknesses. The diffraction intensity ratio of an object thin film to the foundation layer is measured for each object thin film. An analytical line is constructed based on the known film thicknesses of the plural object thin films and the plural diffraction intensity ratios. The film thickness of any sample can be measured based on the measured diffraction intensity ratio and the analytical line. The diffraction intensity ratio method is, however, effective only when the preferred orientation of the object thin film is known. Namely, an analytical line is constructed at first about thin films having the same known preferred orientations and thereafter a film thickness can be determined about any thin film having the same preferred orientation. The second publication says that the diffraction intensity ratio method is not usable for a thin film having an unknown preferred orientation. Consequently, the second publication proposes to use the intensity of a diffracted X-ray from the foundation layer only to measure the thickness of a thin film deposited on the foundation layer. With this method, the thickness of the object thin film can be determined out of relation to the preferred orientation of the object thin film. This method is, however, effective only under the condition that a certain extent of a diffracted X-ray can be observed from the foundation layer. If a certain extent of a diffracted X-ray can not be observed as in the case that the foundation layer is amorphous, the method proposed in the second publication is unusable.
The third publication suggests that the diffraction intensity ratio is measured using two kinds of X-ray wavelengths so that the thickness of a thin film even having a preferred orientation can be determined because the effect of the preferred orientation is cancelled. Stating in detail, the third publication points out that even when the preferred orientation of the sample is changed, there is no relation between the diffracted X-ray intensity and the amount of a film deposited. There are used two kinds of X-ray wavelengths, e.g., the characteristic X-ray of Cr and the characteristic X-ray of Cu, to detect the two intensities of diffracted X-rays in the same direction and from the same lattice spacing. The diffraction intensity ratio of the two intensities depends upon the amount of deposition because the effect of the preferred orientation is cancelled. If an analytical line is constructed in advance about the relationship between the diffracted X-ray intensity and the amount of deposition, the amount of deposition can be determined with the measurement of the diffraction intensity ratio, noting that the relationship would become a curved line. It is noted in this method that it is necessary, for making an analytical line, to prepare plural object thin films having known and different film thicknesses and to measure the diffraction intensity ratio among the object thin films with the use of two kinds of X-ray wavelengths.
The fourth publication discloses that: the diffracted X-ray intensities are measured for both of a sample having a thin film thereon and another sample from which a thin film has been removed; a variation curve of the diffraction intensity ratio with an incident angle being changed is determined, the curve becoming a measured rocking curve; a theoretical rocking curve containing a film thickness and a film density as parameters is produced; and the film thickness and the film density can be determined so that the theoretical rocking curve can approach the measured rocking curve as closely as possible. The fourth publication is deeply pertinent to the present invention in view of the use of a parameter fitting operation about the diffracted X-ray intensity. It is necessary in this method, however, that: the diffracted X-ray intensity is measured for a sample having a thin film deposited thereon; the thin film is removed; and then the diffracted X-ray intensity is measured again for the sample from which the thin film has been removed. Accordingly, this method can not be said to be the nondestructive measurement. Since the greatest advantage of the X-ray-using film thickness measurement method is that it is the nondestructive measurement, the method disclosed in the fourth publication, which must have the step of removing the thin film, would lose the greatest advantage with the use of X-ray.
Noting that the present invention has a feature, which is one of the features of the invention, of using a theoretical formula about the diffracted X-ray intensity in consideration of the orientation density distribution function, the prior art involved with this feature is known and disclosed in H. Toraya, H. Hibino, T. Ida and N. Kuwano, Quantitative basis for the rocking-curve measurement of preferred orientation in polycrystalline thin films, (2003), Journal of Applied Crystallography, 36, p. 890–897 which will be called as the fifth publication.
The film thickness measurement methods disclosed in the second to the fourth publications have the problems described below. The method of the second publication is to measure the thickness of the object thin film based on the intensity of a diffracted X-ray from the foundation layer for the purpose of eliminating the effect of the preferred orientation of the object thin film. This method is effective only under the condition that a certain extent of a diffracted X-ray can be observed from the foundation layer, and accordingly the method is unusable in the case that the foundation layer is amorphous. Explaining with an example, the method of the second publication is unusable for measuring the film thickness of a tantalum layer which is deposited on a tantalum nitride layer, because the tantalum nitride layer is amorphous.
The method of the third publication suggests the measurement of the thickness of the object thin film based on the diffraction intensity ratio which is obtained with the use of two kinds of X-ray wavelengths. This method has problems that (1) an X-ray source which can generate two kinds of X-ray wavelengths must be prepared, for instance, an X-ray tube which can generate the characteristic X-ray of Cr and another X-ray tube which can generate the characteristic X-ray of Cu and (2) an analytical line indicating the relationship between the diffraction intensity ratio and the film thickness must be created beforehand, that is, the prior measurement operation must be carried out for plural object thin films having known and different film thicknesses.
The method of the fourth publication has problems that (1) the diffracted X-ray intensity must be measured again for the sample from which the thin film has been removed after the measurement of the diffracted X-ray intensity for the sample having the thin film deposited thereon, the method being not the nondestructive measurement and (2) a reliable film thickness would not be expected for a sample having a preferred orientation, because the method does not take account of the specific theory for the sample having a preferred orientation.